Beamline will assist cell nuclei research

Working on a shoestring, with nothing but the sense that "this beamline might be useful," researchers at the Wisconsin Synchrotron Radiation Center developed a source of soft X-rays that could spawn a new research area in the damage and repair of cell nuclei.

As reported in the April 24 issue of the journal Science, geneticists used the SRC's soft X-ray source to irradiate one-micron stripes of cell nuclei through a mask. "Typically a half dozen stripes fit into a nucleus, with equal distance between the stripes," says Professor of Materials Science and EngineeringMax G. Lagally. "The repair of DNA damage in the nucleus can now be studied with one-micron lateral resolution."

This novel method allows researchers to examine the damage and repair of DNA in the nucleus of cells. Radiation causes damage to DNA which the nucleus is able to repair if the damage is not too great. Damage occurs when DNA double-strands break. In order to understand the repair processes within the nucleus, sites must be observed within the damaged cells. Lagally, SRC researcher Jim MacKay, and Professors Mike Gould (oncology), Paul deLuca (medical physics) and T. Rockwell Mackie (medical physics) used the synchrotron- generated soft X-ray source and micro-fabricated irradiation masks developed by MacKay to induce DNA damage in discrete subnuclear regions of irradiated cells.

School of Medicine and Public Health Assistant Professor of Medical Genetics John Petrini, working with medical physics PhD candidate Ben Nelms and genetics PhD student Rick Maser, used the soft X-rays and mask to show special repair proteins doing their work by moving immediately from their home bases to remote gene damage sites.

Researchers used the SRC's soft X-ray source to irradiate one-micron stripes of cell nuclei through a mask. The repair of DNA damage can now be studied with one-micron lateral resolution. (large image)

"We wanted to break chromosomes in a tiny area of the nucleus of a living human cell without blasting it away," said Petrini. "The electron accelerator at the synchrotron, which generates so-called `ultra-soft' X-rays that are extremely limited in their effect, allowed us to do that."

The new observation method allows the time dependence of the repair process to be analyzed and opens up a wide range of possible experiments to address fundamental questions of cell damage and repair.

It's ironic that the multi-layer mirror beamline at the SRC may be the only beamline in the world with the proper characteristics to conduct these experiments. The beamline was born out of the scrapped research project of Space Science and Engineering researcher Wilt Sanders.

"Sanders asked if we would consider growing some multilayer films for him for X-ray optics. He was part of a large project to build a soft-X-ray spectrometer to fly on a future space shuttle," Lagally says. "Even though I had known Wilt for many years prior, he did not know of our thin-film deposition capabilities and first contacted another colleague, Phil Cohen at the University of Minnesota, who told Wilt `Why don't you get Lagally to grow them, he is much closer.' So without knowing at all what we were doing, we agreed, because we had the essential facility, a sputter deposition system with two cathodes. We began growing multilayer films in 1985. We used a lot of undergraduates on this project because we had no direct funding."

Because of the Challenger disaster, delays and setbacks, Sanders never built a spectrometer with multilayer mirrors. But Lagally and researchers at the SRC decided to build a monochromator for soft X-rays using multilayer mirrors, and to develop a new soft-X-ray diffraction beamline at the SRC.

"When asked what we would do with it, I did not have an answer," Lagally says. "The wavelengths were too long to resolve atomic distances and all we could hope to see was step distributions or in fact analyze the quality of the multilayer mirrors we were growing. Shows you how wrong one can be."

Lagally and colleagues scrounged used pumps and vacuum chambers and built a beamline for about $20,000, less than one-twentieth the cost of a regular beamline. This low-cost beamline, built on a hunch, has since been used for research resulting in seminal papers on diffuse X-ray scattering and techniques in determining surface roughness in the polishing of silicon wafers.

In 1995 and '96 the beamline was used to measure the quality and uniformity of transmission filters for a spectrometer to be flown by NASA. Lagally says the beamline was the only instrument in the world capable of making those measurements.

In 1996 the multilayer mirror beamline was the essential facility that allowed the SRC to develop an entirely new measurement of the magnetic diffuse scattering at magnetic/nonmagnetic interfaces. Lagally says the degree of "magnetic roughness" is essential for giant magnetoresistance in magnetic films and multilayers.